1. Field of the Invention
The present invention relates to a piezoelectric device, a piezoelectric actuator using the piezoelectric device, and a liquid discharge device using the piezoelectric device.
2. Description of the Related Art
Currently, demands for downsizing of piezoelectric devices, as well as semiconductor devices, are increasing with the development of the microprocessing and micromachining. For example, the piezoelectric devices constituted by a piezoelectric body and electrodes are used as actuators installed in inkjet recording heads. In the piezoelectric devices, the piezoelectric body expands and contracts in correspondence with increase and decrease in the strength of an electric field applied from the electrodes to the piezoelectric body. In order to improve the image quality, it is necessary to increase the installation density of piezoelectric devices constituting each inkjet recording head. In the case where high installation density of the piezoelectric devices are required, it is preferable to minimize the thickness of the piezoelectric body used in each piezoelectric device from the viewpoint of the precision of the processing or machining. Specifically, piezoelectric devices having thin piezoelectric films and exhibiting satisfactory piezoelectric performance are currently demanded. In particular, in the case where the piezoelectric devices are used for discharging ink having high viscosity, high piezoelectric performance is required.
Among the piezoelectric devices using a sintered bulk piezoelectric body, the bimorph-type piezoelectric device is known as a piezoelectric device which can achieve high piezoelectric performance by using a conventional piezoelectric material. The bimorph-type piezoelectric device has a structure in which an intermediate electrode is sandwiched by two piezoelectric bodies. When a predetermined voltage is applied to the bimorph-type piezoelectric device, strain is produced in each of the two piezoelectric bodies. Therefore, the bimorph-type piezoelectric device can produce greater strain than the common piezoelectric device which contains only one piezoelectric body. However, improvement in the stability in the piezoelectric performance, the manufacturing efficiency, and the reliability of the bimorph-type piezoelectric devices are currently required.
Japanese Unexamined Patent Publication No. 2004-096069 (hereinafter referred to as JP2004-096069A) discloses a piezoelectric actuator in which a bimorph-type piezoelectric device is formed on a surface of a diaphragm, a driving electrode is arranged between upper and lower piezoelectric layers in the bimorph-type piezoelectric device, and the upper piezoelectric layer has greater thickness than the lower piezoelectric layer. JP2004-096069A also discloses that the deformation stability can be enhanced when the sum of the thicknesses of the upper and lower piezoelectric layers is approximately 20 micrometers. In addition, JP2004-096069A further discloses that when the bimorph-type piezoelectric device is arranged so that the upper piezoelectric layer covers the driving electrode, air discharges and malfunctions caused by short circuit can be prevented, and therefore the reliability can be increased.
Further, Japanese Unexamined Patent Publication No. 2002-205410 (hereinafter referred to as JP2002-205410A) discloses a process for producing an inkjet recording head having a plurality of pressure chambers which contain ink. Each of the pressure chambers is formed with a plurality of walls including side walls. One of the walls is realized by a vibrating surface of a piezoelectric device, so that the pressure imposed on the ink in each pressure chamber is controlled by vibrating the vibrating surface of a piezoelectric device, and the ink can be externally discharged through a nozzle connected to the pressure chamber. The piezoelectric device is constituted by a lamination of planar piezoelectric bodies in each of which internal driving electrodes are arranged.
As indicated in the paragraph <0002> in JP2004-096069A, the piezoelectric layers in the piezoelectric device of JP2004-096069A are sintered bodies. In addition, the paragraph <0022> in JP2004-096069A discloses that the piezoelectric layers are baked in one operation after the piezoelectric layers are shaped together with electrodes. However, the surface roughness of the sintered bodies is great. In particular, when the thickness of a sintered body is as small as approximately 10 micrometers as is the case with the piezoelectric layers in the piezoelectric device of JP2004-096069A, it is difficult to achieve satisfactory uniformness in the sintered body. Further, the surface roughness is likely to become great, since the grain size of the sintered body is as large as approximately several micrometers.
Furthermore, since the layers of the piezoelectric bodies produced in the process disclosed in JP2002-205410A are also sintered bodies, the surface roughness of the piezoelectric bodies in JP2002-205410A is also likely to be great.
In the case where the surface roughness of a piezoelectric body is great, the electric field applied to the piezoelectric body tends to concentrate at the contact points at which the piezoelectric body is in contact with electrodes, so that there is a risk of degrading the piezoelectric body and decreasing the operation lifetime and the operational stability. In particular, when the thickness of the piezoelectric body is small, it is preferable that the surface roughness of the piezoelectric body be as small as possible, since the operation of the piezoelectric device is affected by the degradation of the piezoelectric body. Specifically, when the thickness of the piezoelectric body is 10 micrometers or smaller, the arithmetic average surface roughness Ra is preferably 0.5 micrometers or smaller, more preferably 0.1 micrometers or smaller, and further preferably smaller 500 nanometers or smaller. Therefore, the concentration of the electric field applied to the piezoelectric body is likely to occur in the piezoelectric actuator disclosed in JP2004-96069A, so that there is a fear of being unable to achieve stable piezoelectric performance and reliability.